1. Field of the Invention
This invention relates to a signal transmitting device of an intermediate lens barrel mounted between a camera and an interchangeable lens.
2. Description of the Prior Art
As an accessory device for changing the focal length of an interchangeable lens, for example, from f=50 mm to f=100 mm, there is already known an auxiliary lens such as a rear conversion lens of the type which is mounted between the interchangeable lens and a camera body. When such a rear conversion lens is used, the minimum F-number of the interchangeable lens as a whole including the conversion lens is increased in accordance with the variation in focal length.
Such variation in minimum F-number is caused not only by a change in the focal length of the entire lens system but also by a variation in effective aperture resulting from the mounting of the auxiliary lens. That is, when the effective aperture is decreased by mounting of the auxiliary lens, the minimum F-number of the entire lens system is increased. Further, when a close-up ring or a bellows device or the like is mounted between the interchangeable lens and the camera, the effective F-number is also varied with a variation in image distance.
On the other hand, there is already known an interchangeable lens having on the mount thereof means for generating an F-number signal during open aperture. This F-number signal is applied to and processed by the exposure meter, the shutter speed priority automatic exposure control device or the program automatic exposure control device of a camera. When the aforementioned interchangeable lens and the intermediate lens barrel are mounted on a camera provided with these devices, malfunction would be caused unless the minimum F-number signal of the interchangeable lens is corrected by an amount of variation in F-number and transmitted to these devices of the camera.
For example, where shutter speed priority automatic exposure control photography is to be effected, if a corrected minimum F-number is transmitted to the camera, the display of the effective minimum F-number or the actually controlled F-number within the viewfinder will be wrong. Also, where program control photography wherein the combination of a shutter speed and an aperture value is selected and controlled in accordance with a predetermined program for a variation in object brightness is to be effected, if an uncorrected minimum F-number is transmitted to the camera, there will not be obtained a shutter speed and an aperture value in a prearranged combination. When an intermediate lens barrel has been mounted, both the weight and length of the entire interchangeable lens system are increased and therefore, in the case of photography, the shutter speed should preferably be controlled to a high speed to reduce the influence of a hand vibration or the like, but the combination of a shutter speed and an aperture value obtained in the absence of the aforementioned correction becomes a combination in which the shutter speed is controlled to a lower speed than a prearranged one, and this is particularly not preferable.
Also, where photography is to be effected with a flashlight unit capable of automatic flash output control mounted on such a camera, if photography is carried out with the above-described substantial aperture value transmitted to the flashlight unit, a proper exposure may be directly obtained.
When the minimum F-number of the interchangeable lens is intactly transmitted to the camera with the intermediate lens barrel mounted thereon, it will become necessary to make correction by the use of an ASA dial or the like by an amount of variation in quantity of light occurring when the intermediate lens barrel has been mounted. Accordingly, when intermediate lens barrels having different amounts of variation in quantity of light are alternately used, correction must be made each time, and, when a camera is used with an interchangeable lens alone mounted thereon, the correction must be put back into its original state, and this renders the operation very complex and may cause mistakes in use.
Such inconveniences may also occur when exposure operation or display is effected with the minimum F-number signal of the phototaking lens introduced into the camera.